WASHINGTON — Republican presidential hopeful Newt Gingrich suffered a big drop in support in Iowa with Ron Paul taking the lead weeks before a key caucus in the state, according to a Democratic polling firm.

The Public Policy Polling telephone survey of 597 likely Republican caucus voters in Iowa found Mr. Paul, a congressman from Texas, leading with 23% of the vote, followed by 20% for former Massachusetts Governor Mitt Romney and 14% for Mr. Gingrich.

“Newt Gingrich’s campaign is rapidly imploding and Gingrich has now seen a big drop in his Iowa standing two weeks in a row,” the polling firm, which is affiliated with the Democratic Party, said in a statement.

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Mr. Gingrich’s share of the vote in the past two weeks has gone from 27% to 22% to 14%, in the latest poll, taken Dec 16-18, with a margin of error for the survey of plus or minus four points, it said.

Mr. Gingrich’s personal favorability numbers also fell during the past fortnight from plus 31 to plus 12 to a minus 1 now among Iowa voters polled ahead of the Iowa caucus on January 3, the polling firm said.

Among the other candidates in the race to oppose Democratic President Barack Obama’s 2012 re-election bid, Rick Santorum, Michele Bachmann, and Rick Perry each received 10% of the votes, while 4% went for Jon Huntsman, and 2% for Gary Johnson, it said.

The libertarian-leaning Mr. Paul’s unusual rise to the top of a poll comes amid a strong campaign in Iowa, the pollsters said. But they said his popularity depended heavily on the youth vote and he trailed both Romney and Mr. Gingrich among older voters.

Mr. Gingrich reached the top of the Republican field last month as the favored conservative alternative to former Massachusetts Governor Mitt Romney. But his front-runner status has prompted attacks from rivals that he is an unreliable conservative and influence peddler, particularly over fat fees he earned from Freddie Mac, a mortgage giant tied to the economic recession.